You can imagine the foreign exchange market as a market. If foreign currency wants to enter China, it will be converted into local currency in this market. Everyone has changed, which makes the local currency scarce, so it becomes more expensive and appreciates. In order to curb this situation, we will throw away the local currency, increase the supply of local currency, and reduce the scarcity above, so that the appreciation of local currency will be controlled. Then how do you sell your local currency in the market? You just change your local currency into foreign currency, which means buying foreign currency. So the situation is like what you said, "In order not to let the local currency appreciate, sell the local currency and buy foreign currency."